On December 9th 2024, Kahina Vertchik has defended her PhD thesis entitled “Order and disorder in condensed DNA toroids analysed by cryo-electron microscopy”, under the supervision of Amélie LEFORESTIER and Guillaume BRUNETTI (JEOL Europe SAS).
DNA toroids are liquid crystalline structures formed spontaneously by the condensation of DNA molecules in solution by condensing agents such as spermine 4+. DNA toroids serve as models for understanding chromosome folding, for example in dsDNA viruses and spermatozoa, and for their potential in nano-engineering. This thesis elucidates the fine structure and morphogenesis of DNA toroids based on cryo electron microscopy analyses. Helical correlations between the DNA double helices within the toroids form a ’zipper’. Optimization of these correlations induces molecular rearrangements and leads to polygonal reshaping of the object, with decrease of the DNA helical repeat in high curvature regions. An order-disorder segregation occurs within toroids, with structural defects (DNA ends and “bridge” defects) concentrated in a specific disordered sector. This phenomenon plays a role in optimization of the interactions, including the zipper motif. Lastly, this thesis presents preliminary results in liquid phase electron microscopy to image biological objects in solution, and follow in the future dynamical processes such as toroid morphogenesis. These results contribute to our fundamental understanding of the biophysics of condensed states of DNA.
Congratulations Kahina !